Most of the common causes of liver injury result in cirrhosis. Cirrhosis involves the destruction of normal liver tissue that leaves non-functioning scar tissue surrounding areas of functioning liver tissue, accompanied with the formation of regenerative liver nodules.
Liver damage or injury may have diverse causes. It may be due to viral or bacterial infections, alcohol abuse, immunological disorders, or cancer, for example.
Viral hepatitis, due to Hepatitis B virus and Hepatitis C virus, for example, are poorly managed diseases that afflict large number of people worldwide. The number of species of hepatitis viruses known is constantly increasing. Apart from Hepatitis B and C virus, at least four other viruses causing virus-associated hepatitis have been discovered so far, called Hepatitis A, D, E and G-Virus.
Sometimes, substances which are normally non-toxic can become hepatotoxic when abused, such as acetaminophen (APAP) overdoses and ethanol.
Alcoholic liver disease is another widespread disease associated with chronic consumption of alcohol. Immune hepatitis is a rare autoimmune disease that is poorly managed. Liver injury also includes damages of the bile ducts. Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by destruction of the intrahepatic bile ducts.
Recently liver injury was found to be a side effect of gene therapy, e.g. acute hepatocellular injury characterized by centrilobular hepatocyte necrosis is a major side effect of viral-based gene therapies targeted to the liver (Nielsen et al., 1998; Bao et al., 1996).
Several studies have demonstrated that damage to the liver in diseases such as alcoholic hepatitis, liver cirrhosis, viral hepatitis and primary biliary cirrhosis is associated with T-helper cell-1 (Th1) responses (Nishimura and Ohta, 1999) (Okamoto et al., 1998) (Harada et al., 1997) (Lee et al., 1999) (Baroni et al, 1999). High levels of the FAS ligand and its receptor (CD95) were reported in liver of hepatitis B and C patients (Luo et al., 1997) (Hiramatsu et al, 1994; Okazaki et al, 1996) thus FAS ligand is considered to be one of the major cytotoxic agents leading to hepatocyte apoptosis. FAS ligand and its receptor are also elevated in alcoholic liver diseases, suggesting once again that Th1 cytokines are involved in the autoimmune processes induced in alcoholic hepatitis (Galle et al., 1995; Taieb et al, 1998; Fiore et al., 1999).
The treatment of cirrhosis includes withdrawing toxic agents such as alcohol, receiving proper nutrition including supplemental vitamins, and treating complications as they arise. Liver transplantation is presently the only cure and may help a person with advanced cirrhosis.
In the early stages of cirrhosis, patients are classified as compensated, meaning that although liver tissue damage has occurred, the patient's liver is still able to detoxify metabolites in the bloodstream. In addition, many patients with compensated liver disease present no symptoms. In the later stages of cirrhosis, patients are classified as decompensated meaning that their ability to detoxify metabolites in the bloodstream is diminished and it is at this stage that the following clinical features may present: bleeding esophageal varices, ascites, jaundice, and encephalopathy (Zakim D, Boyer T D. Hepatology: A Textbook of Liver Disease, Second Edition, Volume 1, 1990, W.B. Saunders Company, Philadelphia).
The liver is also unique in that it is the only mammalian organ that can regenerate its biologically functional parenchymal mass following resection or injury, instead of healing with biologically nonfunctional scar tissue.
The ability of a patient to restore the pre-operative hepatic mass following major liver resection is well-known (Hadjis, 1990). Therefore, the capability to induce the regeneration of an adequate functional hepatic mass would be a significant advance that could prevent many deaths from liver failure. The ability to induce or enhance hepato-cell proliferation would allow hepatic malignancies to be resected and facilitate the increase of healthy hepatic tissue. This will prevent the patient's post-operative death from liver failure due to too little remaining functional liver mass. The same holds e.g. for patients suffering from fulminant hepatic failure from toxic, metabolic, or viral causes if the native liver could be induced to regenerate at a rate that would restore adequate hepatic function.
Kokudo et al. (1992) established an animal model to investigate regenerative response of cirrhotic liver after hepatectomy for example by exogenous added factors and particularly by the transforming growth factor-α (TGF-α). In such model, micronodular cirrhosis was established by the simultaneous administration of CCL4 and phenobarbital. Hepatic DNA synthesis (3H thymidine incorporation into DNA) was tested 24 hours after partial hepatectomy in cirrhotic rats in the presence or in the absence of TGF-α treatment (at 0 and 12 hr after hepatectomy).
IL-6 acts not only as a pro-but also as an anti inflammatory cytokine (Jones et al. 2001). The functional properties of IL-6 are extremely varied and this is reflected by the terminology originally used to describe this cytokine (Horst Ibelgaufts' COPE: Cytokines Online Pathfinder Encyclopaedia).
The biological activitities of IL-6 are mediated by a membrane receptor system comprising two different proteins one named IL-6 Receptor (IL-6R or gp80 reviewed by Jones et al. 2001) and the other gp130 (reviewed by Hirano et al, 1994). Soluble forms of IL-6R (sIL-6R), corresponding to the extracellular domain of gp80, are natural products of the human body found as glycoproteins in blood and in urine (Novick et al, 1990, 1992). An exceptional property of sIL-6R molecules is that they act as potent agonists of IL-6 on many cell types including human cells (Taga et al, 1989; Novick et al, 1992). Even without the intracytoplasmic domain of gp80, sIL-6R is still capable of triggering the dimerization of gp130 in response to IL-6, which in turn mediates the subsequent IL-6-specific signal transduction and biological effects (Murakaini et al, 1993). sIL-6R has two types of interaction with gp130 both of which are essential for the IL-6 specific biological activities (Halimi et al., 1995), and the active IL-6 receptor complex was proposed to be a hexameric structure formed by two gp130 chains, two IL-6R and two IL-6 ligands (Ward et al., 1994; Paonessa et al, 1995).
In contrast to the expression of the cognate IL-6R cellular which has a limited cellular distribution (reviewed by Jones et al. 2001), expression of the trans-membrane-spanning gp130 is found in almost all organs, including heart, kidney, spleen, liver, lung, placenta, and brain (Saito et al. 1992).
There are many different examples which show that IL-6 alone does not induce a specific activity unless the soluble IL-6R is administered. For example, IL-6 induces osteoclast formation in cocultures of mouse bone marrow and osteoblastic cells, only when combined with the sIL-6R (reviewed by Jones et al. 2001). Also, although many neuronal cells are capable of producing IL-6, they remain unresponsive to stimulation by IL-6 itself. Differentiation and survival of neuronal cells can, however, be mediated through the action of sIL-6R (Hirota 1996, Martz 1998).
Chimeric molecules linking the soluble IL-6 receptor and IL-6 together have been described (Chebath et al., 1997). They have been designated IL-6R/IL-6 chimera. The chimeric IL-6R/IL-6 molecules were generated by fusing the entire coding regions of the cDNAs encoding the soluble IL-6 receptor (sIL-6R) and IL-6. Recombinant IL-6R/IL-6 chimera was produced in CHO cells (Chebath et al, 1997, WO99/02552). The IL-6R/IL-6 binds with a higher efficiency to the gp130 chain in vitro than does the mixture of IL-6 with sIL-6R (Kollet et al, 1999).
As mentioned above, interleukin-6 signaling is facilitated through the homodimerization of gp130 to the ligand-receptor complex. Intracellular signaling is subsequently triggered via activation of gp130-associated cytoplasmic tyrosine kinases (JAK1, JAK2, and TYK2) and phosphorylation of STAT1 and STAT3 (Murakami et al 1993, Gerhartz et al. 1996). In contrast, the high-affinity receptors of LIF, OSM, and CNTF activate cells by a heterodimerization between gp130 and a gp130-related protein (the LIF receptor) (Davis et al. 1993). Such homo- or heterodimers activate distinct but overlapping patterns of tyrosine phosphorylation through the Jak-Tyk family of cytoplasmic tyrosine kinases (Boulton et al. 1994). This may contribute to the different cellular responses associated with this family of proteins.
Although IL-6 signaling is recognized to be necessary for the induction of transcription factors involved in liver regeneration, the art appears to be controversial with regard to IL-6 administration for liver protection/regeneration. On one hand, IL-6-deficient mice have been shown to have impaired liver regeneration following partial hepatectomy, and liver regeneration could be reconstituted by IL-6 administration (Cressman et al 1996). The beneficial effects of IL-6 administration were reported also in severe pathological conditions of the liver e.g. in a model of ischemia followed by resection and in a model of acute liver injury induced by CCL4 alone (Selzner 1999 and Kovalovich 2000 respectively). Also, the fusion protein of IL-6 and the soluble IL-6R was also shown to be beneficial for the treatment of liver injury (WO99/02552). However, the high doses of IL-6 which were reported to have beneficial effect on liver protection/regeneration (e.g. in the range of 500 to 1000 mcg IL-6/kg) may not be considered feasible vis-à-vis the prospect of unwanted side effects expected using such large doses. On the another hand, it was reported in a an animal model of regeneration after partial hepatectomy in mouse, that only IL-6 fused to the IL-6R (Hyper-IL6), but not IL-6 alone can induced accelerated reconstitution of the liver (Peters et al. 2000).
Thus, new strategies for treating liver damage caused by a wide range of hepatotoxic agents and gene therapy vectors are thus needed, particularly those that promote rapid hepatocyte proliferation.